Recovery of lead from spent batteries is of significant economic importance, both as a source of raw materials and because of the problems of disposal of hazardous wastes. For many years battery recycling plants have attempted to solve the problems associated with the presence of sulfur and chlorides in battery scrap by extensive beneficiation of the spent battery scrap. The batteries were broken, the acid drained, and the remaining material milled to a small size. The crushing and milling liberated most of the paste portion from the grids and crushed the plastic components of the battery.
In a series of hydrometallurgical processing steps the paste was separated from the metallic lead and plastic portions of the battery. Much of the chloride containing plastic along with other non-recyclable plastic, glass, and inorganic components of the battery were separated from the paste and metallics. The material, however, contains a substantial amount of lead as finely divided lead or active material. Despite persistent efforts to remove the lead, sufficient lead remains in this material to prevent disposal in non-regulated landfills.
The standard method of recovering lead values from spent batteries involves smelting the lead bearing portions of the battery in a reverberatory, rotary, blast, or electric furnace using standard pyrometallurgical procedures. These pyrometallurgical processes have several disadvantages or drawbacks.
The main disadvantage of the pyrometallurgical processes is that they operate at elevated temperatures and generate substantial amounts of sulfur dioxide gas as well as volatile dusts. The dusts carry substantial amounts of volatile metals such as lead, arsenic, antimony, cadmium, and the like. The off gases also contain chlorine or chlorides as a result of combustion of chloride containing materials such as separators made of polyvinyl chloride.
With the restrictions of the Clean Air Act, SO.sub.2 emissions from industrial smelting facilities must be reduced to very low levels. Spent lead acid batteries contain a substantial amount of sulfur in the form of H.sub.2 SO.sub.4 from the electrolyte and even more as PbSO.sub.4 in the active material as the product of battery discharge. High volume battery recycling plants handle hundreds of tons of scrap batteries per day. The sulfur content of a spent battery is about 3.9% of battery weight and thus a plant could have an input of many tons of sulfur per day.
To control the SO.sub.2, rotary furnaces tie up most of the sulfur in the battery scrap as a FeS--Na.sub.2 S soda matte, blast furnaces and electric furnaces can tie up the sulfur as a sulfide matte. Reverberatory furnaces can also use iron or sodium compounds to tie up the sulfur in the slag; however, further processing of the reverberatory furnace slag or disposal of the matte or slag may be a problem due to leaching of heavy metals from the soluble components of the slag.
In order to reduce SO.sub.2 emissions, the separated paste has been treated with solutions of alkali materials such as NaOH or Na.sub.2 CO.sub.3 to react with the PbSO.sub.4 in the following reactions: EQU PbSO.sub.4 +2NaOH.fwdarw.Pb(OH).sub.2 +Na.sub.2 SO.sub.4 EQU PbSO.sub.4 +Na.sub.2 CO.sub.3.fwdarw.PbCO.sub.3 +Na.sub.2 SO.sub.4
The resultant "desulfurized" material is recovered as a sludge or filter cake. Despite extensive efforts to wash the sludge and to desulfurize with excess alkali reagents, substantial amounts of sulfur often remain in desulfurized paste as unreacted PbSO.sub.4 or as Na.sub.2 SO.sub.4 retained in the material. The sulfur content of the non-desulfurized paste is about 6%, while that of the desulfurized paste normally contains about 1% total sulfur or less.
In addition to the sulfur, the paste often contains a number of small PVC particles which are not liberated in the plastic removal system. When the desulfurized paste and metallics are smelted in furnaces, however, the SO.sub.2 content of the gas stream is still at elevated levels, thus requiring the addition of fluxes to tie up the sulfur as a matte or soda matte. With desulfurization, only the quantity of these wastes is decreased.
To assure compliance with regulations restricting the emission of SO.sub.2 to low values, battery recycling plants utilizing reverberatory furnaces have installed alkali or lime scrubbers to reduce the amount of SO.sub.2 emitted despite the desulfurization of the feed material. Lime scrubbers generate substantial amounts of gypsum as well as CaSO.sub.3, while alkali scrubbers generate mixed sulfate-sulfite solutions. In addition to the SO.sub.2 the scrubbers also scrub any contained chlorides. The effluent sludge from the lime scrubbers as well as sludge from calcium neutralization of the battery acid is generally sent to landfills.
In processes where the active material (paste) portion of the battery is separated from the metallics and is desulfurized using a solution of ammonia, sodium or potassium hydroxide, carbonate or bicarbonate, lead carbonate or lead hydroxide and relatively pure Na.sub.2 SO.sub.4, (NH.sub.4).sub.2 SO.sub.4, K.sub.2 SO.sub.4, etc. solutions are produced. These solutions are often crystallized to recover the sulfate salts.
When alkali scrubbers are used to recover sulfur, a discharge solution containing mixed sulfate, bisulfite, thiosulphate, sulfite, and other sulfur species along with chlorides and heavy metals is produced. Because of the chlorides and heavy metals, the scrubber solutions after oxidation to sulfate have not been able to be processed into saleable sulfate products. These alkali sulphate solutions, when cleaned of heavy metals and where the level of total dissolved solids permits, have been discharged as waste water into sanitary sewers.
Where the disposal of high levels of dissolved solids into the waste water is not possible, lime scrubbers have been used to remove the sulfur from the furnace off gases. In these scrubbers the sulfur is trapped as CaSO.sub.3, CaSO.sub.4, or mixed sulfur compounds. When oxidized to gypsum, the material has low solubility in the scrubbing solution. Because the scrubber products are not soluble, fouling of the scrubber interior is a major problem. In addition, the gypsum produced from scrubbers of battery recycling is a solid waste and may be a hazardous waste depending on the heavy metal content of the material. The gypsum is also produced as a sludge which can restrict disposal.
An additional problem is the chloride which can form soluble CaCl.sub.2 and build up in the scrubber solutions. These chloride solutions are very soluble and present problems of high dissolved solids in waste water discharges. An additional problem is small amounts of magnesium in the lime. Magnesium reacts with the SO.sub.2 or Cl to form soluble magnesium salts which compound the dissolved solids problem of lime scrubber discharges.
The effluent from alkali scrubbers in general cannot be utilized to produce a sulfate product due to the presence of heavy metals and chlorides scrubbed from the gas stream. When cleaned of heavy metals the solutions must be disposed of in sewers despite the high salt content. Many municipalities have restricted the total dissolved solids in the plant effluent, thus reducing the ability of the plant to discharge these scrubber solutions.
In contrast to the prior art methods, the method of the present invention assures that greater than 99% of the sulfur in the battery is recovered and the heavy metal content, SO.sub.2, and chloride content of the off gases is reduced to negligible values.